Naltrexone has the chemical name morphinan-6-one, 17-(cyclopropylmethyl)-4,5-epoxy-3,14-dihydroxy-(5α). The molecular formula of naltrexone is C20H23NO4 and its molecular weight is 341.41 in the anhydrous form (<1% maximum water content). The chemical structure of naltrexone is shown below.

Naltrexone has been approved for use in the treatment of alcoholism or narcotic addiction. It is believed that naltrexone functions by blocking the brain receptors that trigger the effects of alcohol or narcotics. Naltrexone is marketed by Durmed in the form of a tablet under the tradename ReVia® and by Alkermes in the form of a powder for injectable suspension under the tradename Vivitrol®.
Naloxone has the chemical name (−)-17-allyl-4,5α-epoxy-3,14-dihydroxymorphinan-6-one and the molecular formula C19H21NO4. The chemical structure of naloxone is shown below.

In the early 1980s, it was reported that the administration of low dose naltrexone (less than 10 mg naltrexone per day) increases the production of endogenous endomorphins, especially the endogenous pentapeptide metenkephalin, and increases the number and density of metenkephalin receptors by intermittently blocking opiate receptors. (I. S. Zagon & P. J. McLaughlin, “Naltrexone modulates tumor response in mice with neuroblastoma”, Science, 221: 671-3 (12 Aug. 1983). This increase in metenkephalin is believed to enhance homeostatic regulation of the natural immune function of the human body.
In view of Zagon's findings, Bernard Bihari reported the use of low dose naltrexone for the treatment of patients with AIDS (U.S. Pat. No. 4,888,346) and herpes (U.S. Pat. No. 5,356,900). Further, Nicholas Plotnikoff reported the use of low dose naltrexone for the treatment of herpes, HIV infection, cytomegalovirus, coronavirus, influenza A and Japanese encephalitis. (E. A. Moore & S. Wilkinson, THE PROMISE OF LOW DOSE NALTREXONE THERAPY: POTENTIAL BENEFITS IN CANCER, AUTOIMMUNE, NEUROLOGICAL AND INFECTIOUS DISORDERS (McFarland & Company, Inc., Publishers, 2009)).